1. Field of the Invention
The present invention relates to spring brake systems for heavy duty vehicles, and in particular to a removable, breathable filter cap for attachment to the emergency brake housing of a spring brake actuator which prevents moisture and unwanted contaminants from entering the actuator, while allowing air to pass into and out of the actuator during normal use.
2. Description of the Prior Art
Spring brake actuators are widely used in the trucking industry to control the brakes of heavy duty vehicles and their trailers. There are two basic types of spring brake actuators, service brake actuators, and emergency brake actuators. These actuators can be deployed as separate assemblies, or combined into a single dual assembly including both a service brake and an emergency brake actuator. A service brake actuator is operator controlled and is used for slowing or stopping a vehicle. An emergency or parking brake actuator is typically held off with air pressure while the vehicle is in use, but automatically engages when air pressure is removed, such as when the vehicle is turned off or if there is a failure in the vehicle air system.
A typical service brake actuator is characterized by a closed housing which contains a movable diaphragm stretched across the inside. One side of the diaphragm moves a slidable push rod which extends out of the housing for attachment to the brakes of the vehicle. On the other side of the diaphragm a sealed chamber is formed within the housing. An opening is provided in the sealed service brake chamber for connection to a pneumatic (air) pressure source usually provided by an on-board air compressor. The brakes of the vehicle can be applied by introducing sufficient pneumatic pressure into the sealed chamber to act against the service brake diaphragm which moves the push rod out. A small return spring is ordinarily provided inside the service brake housing around the push rod to urge it to retract when the air pressure behind the diaphragm is reduced.
A typical emergency brake actuator is attached in axial alignment with or made a part of the service brake assembly. The emergency brake is a separate closed housing which contains a heavy main compression spring and a second movable diaphragm creating a second sealed chamber. The emergency brake diaphragm is also attached to or directly associated with the slidable central push rod of the service brake. The second sealed chamber is formed inside the emergency brake housing on one side of the diaphragm, and the heavy main compression spring is deployed on the opposite side. As with the service brake, the sealed chamber of the emergency brake is connected to the on-board pneumatic source of the vehicle. As long as sufficient air pressure is provided to the sealed chamber, the diaphragm in the emergency brake will remain fully expanded thereby compressing (caging) the main spring. However, should air pressure fall, or should there be a leak in the sealed chamber, the diaphragm will be unable to hold the main compression spring in place. When this occurs, either slowly or quickly, the main compression spring will expand causing the push rod to be extended out thereby applying the brakes of the vehicle.
Under normal conditions, when the vehicle is parked, the air pressure to the emergency brake is removed causing the main compression spring to apply the brakes. As the emergency brake is engaged and disengaged, air pressure is introduced and removed from the lower sealed chamber in the emergency brake housing, causing the diaphragm inside the housing to move back and forth. In order for the diaphragm to expand and compress the main spring, the air in the upper housing around the spring must be allowed to escape to the outside. Then, in order for the diaphragm to contract and release the main spring, air must be allowed to enter the area of the upper housing around the spring. Typical emergency brake housing assemblies include a plurality of air holes in the upper housing in the vicinity of the main spring to allow air to enter and exit this region as the diaphragm moves back and forth. The unfortunate effect of this action is to allow dirt, water, oil, debris, and other unwanted materials to enter the emergency brake housing from the outside through the air holes. Several inadequate or cumbersome solutions have been advanced to respond to this problem.
The invention of U.S. Pat. No. 4,508,018 describes a sealed internal breathing system which utilizes a tube connected between the service brake housing and the emergency brake housing. Air is transmitted between the two sealed housings through the tube, so that no outside air is ever needed. However, the use of such a tube requires specially designed housings for both the spring brake and emergency brake making it difficult and costly to manufacture.
The invention of U.S. Pat. No. 5,320,026 employs a rubber dust guard in the upper housing between the power spring and the housing wall to prevent particulate matter from passing beyond the spring into the actuator tube where it might affect the sliding rod. However, this invention does not prevent foreign matter from entering the upper chamber in the first place through the air openings. Thus, foreign material can collect around the power spring potentially affecting operation of the brake.
The inventions of U.S. Pat. Nos. 5,372,059 and 5,722,311 use a pair of valves: a first one-way valve allows air to enter the area around the main spring only from the sealed chamber in the spring brake housing; another other one-way valve only allows air to escape from this area to the outside. A similar approach is taught in U.S. Pat. No. 5,873,297 which employs a double check valve in the housing, and a separate valve in the upper housing diaphragm for communication between the sealed chamber and the area around the main spring. Each of these inventions requires cumbersome specially designed emergency brake housing structures including at least one opening(s) in the spring brake diaphragm, together with the use of no less than two separate valves, leaving such systems vulnerable to several different kinds of failures which could be costly and difficult to repair.
The invention of U.S. Pat. No. 5,836,233 describes a spring brake actuator which employs breather holes located on the cylindrical side walls of the upper housing such that when the main spring is compressed or caged, the diaphragm covers the holes. However, when uncaged, these unfiltered breather holes are exposed to the atmosphere, allowing foreign material to enter the housing. A check valve is also provided in a cap attached to the upper housing.
A recent device utilizes a hydrophobic filter system using a Gore-Tex(copyright) membrane which is integrated into the top of the emergency brake housing to prevent water and contaminants from entering the housing while allowing air to flow in and out. However, because this filter system is integrated into a caging device (used to manually compress the main spring) it is impossible to remove or change the filter should it become clogged. The Gore-Tex(copyright) material does not breathe well, and may clog easily. Moreover, the integral caging device is bulky and requires more space under the vehicle than a brake without such a device, thereby making such a brake assembly unusable on vehicles with restricted available space.
The present invention overcomes the drawbacks of the prior art while still enjoying the benefits of a breathable upper housing for an emergency spring brake actuator by providing an easily removable and replaceable filter cap for attachment the top of the upper housing which includes a plurality of ventilation holes and a membrane through which air is allowed to pass into and out of the emergency brake housing during normal use. The membrane itself is preferably made of an oilophobic and hydrophobic material (not necessarily Gore-Tex(copyright)) in order to prevent oil, water and debris from entering the housing. The filter cap of the present invention may be removably inserted into the upper caging opening of the emergency brake actuator. This opening is provided in many actuators in order to allow a caging tool to be inserted into the actuator to cage (compress) the main spring when no air pressure is available. As a result, the present invention may be easily adapted for use on any of a large number of existing emergency brake housing assemblies at minimal cost, thereby avoiding the need for such costly, cumbersome and unnecessary structures as valves, tubes, perforated diaphragms and/or integrated caging apparatus in the brake actuator.
Many existing brake actuators employ a plurality (e.g. four or more) of unfiltered air openings in the upper housing. When the filter cap of the present invention is installed, these openings are plugged so that all air is filtered through the membrane in the cap. In an emergency or service situation, the cap may be easily removed to allow a caging tool to be inserted into the upper housing to release the emergency spring brake. After removal, the cap may be reused or replaced.
Several alternative designs for the filter cap of the present invention are available. In the preferred design, the cap includes an annular flange which snaps into the caging tool opening at the top of the emergency spring brake housing. The filter (which includes the membrane and breathing holes) is a separate piece which snaps into the large central opening in the cap. The cap has two oppositely positioned extensions thereon: the first is an anchor piece which includes a plug for insertion into an existing breathing hole on the housing, and the second is a ribbed grip tab which is pulled in order to remove the cap from the housing.
Alternative designs for the present invention include: (a) the filter being integrated into the cap as a single unit; (b) the filter being screwed over external threads in the cap; (c) the filter being screwed into internal threads in the cap; and (d) one or both of the extensions being removed.
It is therefore a primary object of the present invention to provide a removable breathable filter cap for use on the upper housing of an emergency spring brake actuator which prevents foreign material from being introduced into the housing from the outside.
It is also a primary object of the present invention to provide a breathable filter cap that may be attached into the opening for a caging tool located on the upper housing of an emergency spring brake actuator.
It is a further important object of the present invention to provide a low cost breathable filter cap that may be attached to the upper housing of any of a number of emergency spring brake actuator designs without any modification to the actuators themselves.
It is a further object of the present invention to provide a breathable filter cap for attachment to the upper housing of an emergency spring brake actuator in conjunction with plugs that are inserted into the breather holes of the housing.
Additional objects of the invention will be apparent from the detailed descriptions and the claims herein.